1. FIELD OF THE INVENTION
The present invention relates to a device for converting input frequencies into digital values.
2. DESCRIPTION OF THE PRIOR ART
In the conventional frequency detection of the digital type, pulses having a fixed repetition rate are counted during a period of the input signal to be measured and the digital output value is obtained by inverting the value of the count. In this detection system, there is a certain proportional relationship between the input frequency and the output digital value. For example, when the input frequency is doubled, the output value is also doubled and the tripled input frequency results in the trebled output value. Sometimes, there is a need for a case where the digital output value varies in response to the change in the input frequency, not in a linear manner but in accordance with a predetermined characteristic. There has been in the prior art no such device as meets the above mentioned requirement.
Such a device will be described briefly by way of an example. Now, let it be assumed that a pulse is delivered each time the crank shaft of the engine on an automobile rotates through a predetermined constant angle. So, by detecting the repetition rate of such pulses, the rotational speed of the engine can be determined. In case where the rotational speed is used as the factor to determine the amount of fuel injected into the combustion chamber of the engine, the precision of fuel injection control at high rotational speeds differs from that at low rotational speeds. In a high rotation range, e.g. near 6000 rpm, the rotations at 6000 and 6200 rpm's can be maintained with almost the same amount of injected fuel. On the other hand, in a low rotation range, e.g. near 1000 rpm, the rotations at 1000 and 1020 rpm's must be kept respectively with different amounts of injected fuel. This is because the volume of inlet air is considerably changed by a small change in the opening of the throttle valve in the low rotation range while in the high rotation range the volume is not so much changed by rather a large change in the opening.
Thus, the conventional device with a fixed detection accuracy cannot be applied to the case where different detection precisions are needed for different ranges of rotational speeds. In order for the conventional detection device to be adapted for such a case, the device must be provided with an additional circuit having a desired characteristic, which can correct the output of the counting circuit to a predetermined accuracy. With this additional circuit, however, the digital-digital conversion section must be also provided with an additional means for obtaining the desired characteristic. Hence, there is caused a drawback that the overall structure is complicated, the reliability is degraded and the production cost is increased.
If the change in the output digital value, following the change in the input frequency in various manners can be previously set, then the change in the output digital value corresponding to the change in the input frequency can be made large for a frequency range where high accuracy is needed. And under the same circumstances, the change in the output digital value in response to the change in the input frequency can be made small for a frequency range where so high an accuracy is not required.